Method of surface treatment surface treatment of void containing substrate

ABSTRACT

To a first organic liquid which has dissolved therein a flurocarbon polymer, e.g. 5 to 50% w/w e.g. PVDF, is added a second, compatible, non-solvent organic liquid in an amount (e.g. 1-50%) w/w such as to leave the polymer in solution. The mixed solution is applied to a leather or fabric substrate and heated e.g. at 60° C. to 150° C. for 2-15 minutes. Polymer deposits as the first liquid evaporates and pores form therein as the second liquid evaporates from the deposit. The anchored layer produced repels liquid water, but allows passage of water vapor.

BACKGROUND OF THE INVENTION

This invention relates to the treatment of substrates to prevent or atleast greatly diminish the penetration into or through the substrate ofliquid water while still permitting the passage into or through thesubstrate of water vapour.

The invention may be embodied as a method of treatment of the treatedsubstrate itself, whether separate and capable of further fabricationinto a final product or whether part of a permanent structure.

While the invention can be applied, for example, to the treatment offabrics or the treatment of vapour-permeable layers such as natural orsynthetic leather layers it can also therefore be applied to thetreatment of a structural or decorative surface which needs to bepermeable to water vapour while repelling or deterring the ingress ofliquid water. In other words, the invention can be applied to thewater-proofing of fabrics, to the surface treatment of leather, and tothe surface treatment of walls, floors and ceilings to confer waterrepellency.

For ease of description the invention will be discussed primarily interms of treatment of fabrics to confer water repellency.

It is well known to provide in apparel as a protective layer againstrain, or against water droplets in the form of mist, an impermeablelayer of polymer or the like. While suitable for short term wear,completely impermeable articles eventually generate considerablediscomfort for the user, since the inevitable water vapour generatedfrom the skin of the user cannot escape, accumulates at the inside ofthe impermeable layer and condenses. The garment becomes humid anduncomfortable to wear and becomes wet inside.

Because of this it is commonplace to treat fabrics to permit more orless passage of water vapour while preventing, or hindering, passage ofwater droplets. The water vapour molecule is many orders of magnitudesmaller than a water droplet, and can pass through fine orifices whichsurface tension does not allow a water droplet to penetrate.

One known method of providing a "breathable" but water-repellent fabricis to utilize a fabric the threads of which expand upon contact withwater. In normal use, the interstices between the weave are open enoughto present no barrier to the passage of water vapour. If, however, thefabric is contacted with water the individual threads swell and tend toclose off the gaps between them. There is still some facility forpassing water vapour through such fabrics but the swollen moist fabricthen prevents substantial transfer of water as liquid through from theoutside to the inside.

There are, however, obvious disadvantages with such fabrics since theydo become wet in use. Efficiency is not high, either for vapourtransmission outwards or for the prevention of water travelling inwards.

A considerable improvement upon such materials can be obtained byconventional waterproofing, providing a water repellent coating for thefibres. This is the basis of the majority of waterproof materials, andinvolves treating the fabric in a solution of a water repellent polymer,typically a natural or synthetic rubber, so that a coating of suchrubber is formed over each yarn or fibre. To some extent this has theeffect of decreasing the size of the interstices between the fibres, butstill leaving orifices through which water vapour can move inwards andoutwards. However, the water repellent nature of the coating, definingthe limits of these small interstices ensures that droplets of water donot penetrate the material (unless in extreme conditions). Water istherefore deflected while preserving the inside of the garment in a drystate. The differences between this and the foregoing proposal are thatthe threads themselves do not become wet and swollen, and that water isturned away from the fabric by the water-repellent coating on thethreads.

A further improvement in fabric characteristics can be obtained by aprocess involving a different type of water-repellent polymer. If a filmof polytetrafluoroethyethylene is produced e.g. by extrusion or casting,and thereafter stretched, it breaks up as an irregular network offibrils while still retaining its essential nature as a sheet. Insteadof stretching and tearing, it undergoes innumerable small fractures andcracks. It is known to take a sheet of such fibrillated or fracturedfilm and to sandwich it between two layers of fabric, or possibly toadhere it only to a back of the fabric layer. Such a sandwich, or such atwo-layer composition, is water repellent by virtue of the high waterrepellency of the PTFE. Water droplets, whether as rain or mist, are fartoo large to penetrate the small orifices between the fibrillatedmaterial, but of course these orifices, although small, present nobarrier to the outward passage of water vapour molecules. Moreover, thefabric is free from any rubbery texture or smell, and the material hasthe advantage that it can be made up as a laminate from suitable reelsof fabric and film, without the necessity for a dipping and curingprocess.

The present invention represents a new category of process forconferring water repellence upon a fabric while retaining vapourtransmitting properties. It involves both the use of a water-repellentpolymer, (such as a fluoropolymer) and the use of a dipping process, andgives a product where such polymer is present in a different physicalstate from anything envisaged or attainable by the prior art.

It is known in a totally different technical field to provide anon-stick coating upon for example a cooking utensil by sintering to theutensil PTFE powder previously applied as a suspension to the articleand dried to a layer. It is also known, as an improvement or variant ofthis sintering process to provide a specialised curable paint vehiclecontaining particles of fluoropolymer with non-stick properties, andadhering this to a surface by curing the paint rather than by sinteringthe particles. In each instance, however, the objective is to obtain agood coating of material free from voids which can of course alter thenon-stick properties and serve as sites for initiation of break up ofthe film. To this end, the surfaces to be treated are themselvestypically quite smooth prior to application, so as to form a suitablebasis for any provision of a non-stick sintered or cured layer.

SUMMARY

The present invention involves the treatment of fabric with particulatewater-repellent polymer and the conversion of such particulate materialinto an adherent and coherent polymer layer by heat treatment, thepolymer layer comprising a multiplicity of voids or orifices permittingthe passage of water vapour and not permitting the passage of liquidwater.

By contrast therefore, whereas the prior art was concerned with taking aprepared surface and providing thereupon a good integral layer free fromvoids, the present invention is concerned with taking an irregularsubstrate such as a fabric and providing in and upon the fibres of thefabric a coherent layer but of an "imperfect" i.e. perforate nature.

In one aspect the invention describes a fabric having supported thereonand disseminated throughout at least a surface thickness of thestructure thereof a layer of water-repellent polymer including amultiplicity of orifices. A wide range of fabrics can be used; a closewoven nylon fabric of 80-140 g.s.m. is a typical example.

The material of the present invention is to be distinguished from knownmaterials in which a separate layer is attached to the fabric. Whilethis layer is supported by the fabric, it is not disseminated throughoutthe structure but merely exists as a separate backing or sandwichedlayer.

In another aspect the invention relates to garments, more especiallygarments having requirements of water repellency and vapourpermeability, consisting at least in part of the fabric as definedabove.

The invention also provides a layer of flexible porous material (such asartificial or natural leather) having at and within a surface thicknessthereof a coherent but perforate layer of a water-repellent polymer.

The invention further envisages building structure the surface of whichhas coated thereupon a perforate layer of polymer material closelyfollowing the pores and irregularities of the surface whereby liquidwater cannot penetrate into or through the wall but water vapour canpass outwards through the said layer.

In one aspect the invention consists in a method of providing awater-repellent but water-vapour-permeable coating over avoid-containing substrate, comprising the steps of taking a solution ofa fluoropolymer in a mixture comprising at least a first organic liquidwhich is solvent thereof and a second organic liquid non-solvent for thefluoropolymer and of lower volatility present in an amount such that nofluoropolymer is precipitated; applying the solution to at least onesurface of the said substrate; and heating the substrate to drive offthe mixture of liquids as vapour, whereby initial loss of first(solvent) organic liquids(s) as vapour causes fluoropolymer to depositwithin the voids of the substrate and over the surface thereof andsubsequent loss of second (non-solvent) organic liquid(s) as vapourcauses at least in part formation of pores through the deposited surfacelayer.

The void-containing substrate may be solid structural or claddingmaterial e.g. concrete, brick, plaster, masonary, chipboard, hardboard,paperboard, or composite board, having a porous nature such as to absorbliquid water into its surface and eventually transmit liquid waterthrough its thickness. The discontinuties or voids may be inherent orimparted as part of the production process. Alternatively, the substratecan be a protective layer to furnishing or apparel, e.g. of natural orsynthetic leather, or of a foamed polymer or rubber base, as used inupholstery, camping equipment, motorvehicles, boots, shoes, gloves etc.In particular it may be a fabric, either a heavy-duty canvas or sheetingmaterial (e.g. for camping, military or transport use) or a lighterwearable fabric as used for rainproof or shower-proof garments. Thevoids or discontinuities in such instances are the gaps between the warpand weft yarns (or in the knitted fabrics) and the gaps betweenindividual fibres of which the yarns are composed.

Fabric coating is a major aspect of the invention, and a typical productcomprises the original fabric within the various gaps or discontinuitiesof which is located a porous anchoring material of the fluoropolymer andupon at least one surface of which, integrally connected with thematrix, is a porous surface layer of the fluoropolymer. Usually,although not necessarily, the matrix has larger, ragged pores orinterstices and a single surface layer is present as a clearlyindentifiable separate layer with smaller through pores and/orinterconnecting cells open to the inner and outer faces.

The fluoropolymer of the present invention are homopolymers orcopolymers (including terpolymers and higher) containing a carbon chainbackbone to the atoms of which are attached either fluorine or afluorinated substituted group.

Hydrogen, or other halogen, substituents may also be present to minorextents. Fluoropolymers are well known in the chemical art, and examplesinclude polytetrafluoroethylene (PTFE) polyhexafluoropropylene (PHFP),polyvinylidenefluoride (PVDF) and the like. Moreover, numerousfluoropolymers are sold under Trade Names such as TFB 7200, the lesssoluble TFB 7100 (both by Hoechst containing and TFE units, HFP unitsand VDF vinylidene fluoride units) KYNAR (PVDF by Pennwalt) and LUMIFLON(a fluoropolymer solution including vinyl ethers and produced bysolution polymerisation by Asahi) and are distinguished by theirproperties rather than by a disclosure of detected chemical composition.

The first organic liquid(s) can be any which is solvent for thefluoropolymer or can indeed (as in the case of LUMIFLON) be the liquidsolvent of the solution as purchased i.e. xylene. It should be of highervolatility than the second liquid i.e. lower boiling point. Typicallylow-boiling solvents which may be used include di-aliphatic ketones(especially those ketones wherein neither alkyl-group exceeds six carbonatoms, a preferred example being methyl isobutyl ketone, MIBK),di-aromatic ketones of similarly low molecular weights and boilingpoints, alkylaromatic ketones, or formamides such as dimethylformamideor pyrrolidones. The amounts of such solvents which may be used arevariable depending on the starting polymer and other components, andproperties desired for the coating. In general, the amount of polymerpresent in the solution with the first liquid, on a dry weight basiswill range from say 5% to 50%, less being wasteful of solvent and ofdrying energy, and more giving difficulties in solution and application.More usually, from 15% to 30% of polymer is present relative to thetotal of the first liquid(s) and polymers.

The second, i.e. non-solvent, organic liquid(s) must be compatible withthe first liquid(s), at least up to a reasonably high level ofincorporation and must be less volatile e.g. have a higher boilingpoint. There appears to be two classes of such liquids particularlysuitable for use, and especially when the first liquid is a ketone,namely (a) higher-boiling aliphatic or aromatic hydrocarbons, ormixtures thereof e.g. from C₈, or more especially C₁₂, and above or (b)hydroxy-substituted aliphatic hydrocarbons, possibly higher alcoholssuch as decanols, but more especially glycols such as ethylene glycol,and the lower liquid, polyethylene glycols e.g. up to PEG 400.

The amount of such second organic liquid(s) to be used is, in thepractice of this invention, widely variable. It should not normally beso much as to precipitate the fluoropolymer before the mixture iscontacted with the substrate. There should, however, be enough that thefluorocarbon will precipitate out as the coating is heated and that thevapour will at least assist in producing the necessary porosity at thesurface. In general, from 1% to 50% w/w (of second liquid(s) to originalsolution) is used, more preferably 5% to 20%.

The man skilled in the chemical art will realise that more than oneprocedure is available for making up the initial solution in the mixtureof liquids. The preferred procedure is to make up a solution in the saidfirst liquid, and thereafter add the second liquid in an amountinsufficient to bring about precipitation in the liquid phase.

In the practice of the invention it is preferred to heat the substrateto a temperature between 60° and 150°, and more usually between 75° and120° C. Depending to some extent on the temperature and solvents used,from 2 to 15 minutes will generally suffice to drive off both liquids asvapour; enough time should be allowed for escape of the second, lessvolatile, liquid but the upper limit does not appear to be particularlysensitive. That is to say, the deposit, once formed, appears generallystable to further temperature exposure. In most instances from 3 to 10minutes is adequate for drying off of solvents.

Another aspect of the invention consists in a fabric substrate carryingas a protective layer against passage of liquid water the integralcombination of a porous anchoring matrix of fluoropolymer locatedbetween the filaments of, and within the gaps between, the fabric yarnand a porous surface layer over at least one surface. The inventionextends to such fabric in the piece or made up as weatherproof articleswhether of apparel e.g. raincoats, clocks, gloves, leggings or other usee.g. tents, covers of the like.

The mechanism of the process of the invention is not fully elucidated,and may indeed partake of different features with different startingmaterials and operating conditions. Thus, drying-off of the secondliquid(s) may merely lead to precipitation of flurocarbon particles,which agglomerate with void-formation upon subsequent heating (i.e.rather as an imperfect sintered coating on a solid substrate). It seemsmore likely, however, that some gelation or softening of the precipitateis present, so that the agglomeration takes place before heating, andvapours are driven off and through the precipitated material to providethe necessary pores. Indeed, some formulations may tend towards uniformfilm-formation, all of the porosity being induced by vapour escape atdifferent stages of coating formation. Thus, with fabric coating, it maybe advantageous to at least partially seal off one face (e.g. on aprocess roller) during at least part of the heat treatment, to modifypore formation. However, any theory as to operation should not affectthe scope of the claims appended hereto.

A particular embodiment of the invention is now described below by wayof example only, and with reference to the single FIGURE of accompanyingdrawing showing a diagrammatic side view of coating equipment.

EXAMPLE

20 gms of the fluoropolymer TFB 7200, supplied by Hoechst Chemicals aredissolved in 80 gms of methyl iso-butyl ketone (MIBK) to give atheoretical solids content of 20%. Once completely dissolved by highspeed mixing the solution is stored in a sealed container for 4 hours toallow for de-aeration.

This solution when free from air bubbles has a viscosity of 260 secs DIN4 cup at 20° C.

To 20 gms of this solution between 0.5 and 2.0 gms (typically 1.0 gm) ofliquid paraffin are carefully added with vigorous stirring to preventpremature precipitation of the TEB fluoropolymer. To this solutionbetween 2.5 grms of monethylene glycol, is added, again with constantagitation. This solution is sealed, and stored for 4 hours prior to use,to enable de-aeration to take place. After this period, a typicalviscosity would be 200 seconds DIN 4 at 20° C.

For laboratory purposes, a sample of a close woven nylon cloth of 120g.s.m. is stretched between in a holding jig in the manner shown inFIG. 1. It is important that the cloth is free from grease and aerialcontamination, and this is achieved if severe by a commercial "genklene"wash, followed by a thorough air purge using dry air at 10 psi pressure.

Depending on the cloth density, deposition of the wet film is by K-handcoater bars (RK Print-Coat Instruments Ltd., Royston, Herts) using the100 μ bar No. 8. It is necessary to draw down an even coating onto thecloth to prevent the formation of air bubbles. The coated sample isimmediately placed into an oven having an air temperature of 110° C.,for 3 minutes.

The dry coating weight was 28 g.s.m. The pressure required to forcewater through the cloth was greater than 15 p.s.i. The evaporation lossthrough the film at 36° C. into an atmosphere at 18° C. and 30%humidity, over 24 hours was about 3000 g per sq. meter of fabric.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a side view of suitable coating equipment is used for theabove example. Fabric 1 is tensioned by clamps 2, 3 at the upper face ofa holding jig 4. The K-coated bar 5 is drawn over the fabric surface inthe direction of arrow A. The wet coating material 6 is thus spreaduniformly over the fabric 1.

The above example can be modified and the component proportions whichcan be used, or characteristics achieved, can vary over a range ofvalues. For example, from 0.5 to 5 g. of the glycol may be used, and theviscosity achieved can range from 150 to 300 seconds DIN 4 at 20° C.Fabrics of 80-140 g.s.m. weight have been coated, and an 80 μ coatingbar has also been used. The oven temperature has been varied between 85°C. and 190° C., and the time residence from 30 seconds to 5 minutes. Auseful range of dry coating weight is from 25 to 40 g.s.m. and theevaporation loss can range from 2800 to 4000 g/m² /24 hours under theconditions specified above.

EXAMPLE 2

20 grms of fluoropolymer TFB 7200 supplied by Hoechst Chemicals aredissolved in 80 grams of methyl isobutyl ketone (MIBK) to give atheoretical solids content of 20%. Once completely dissolved by highspeed mixing the solution is stored in a sealed container for 6 hours toallow for de-aeration.

This solution when free from air bubbles has a viscosity of 269 secsDIN4 cup at 20° C.

To 100 grms of this solution between 10 grms of mono ethylene glycol arecarefully added with vigorous stirring.

For laboratory purposes a sample of a close woven nylon cloth of 100gms/sq. meter is stretched in a holding jig in the manner shown inFIG. 1. It is important that the cloth is free from grease and aerialcontamination.

Depending on cloth density deposition of the wet film is by K-handcoater bars, selecting the correct bar to give a dry coating weight ofbetween 20 and 25 g.s.m. The coated sample is immediately placed in anoven at an air temperature of 80° C. for 5 minutes. At this coatingweight the pressure required to force water through the cloth was 15psi.

The evaporation rate through the film 24° C. into an atmosphere at 18°C. and between 50 and 60% relative humidity over 24 hours was of theorder of 2500 g per sq. meter of fabric.

EXAMPLE 3

20 grms of PVDF (commercial name Kynar, supplied by Pennwalt CompanyLimited) was dissolved in 80 grms of dimethyl formamide and allowed todeaerate in the manner of Example 1.

To 100 grms of this solution between 8.5 grms of mono ethylene glycolwas added with vigorous stirring. (Alternatively both dimethyl formamideand mono ethylene glycol can be pre-mixed and the PVDF solid slowlyadded again with vigorous stirring).

The cloth was then coated in a similar manner to Example 1 andimmediately placed in an oven at an air temperature of 150° C. for 5minutes. Cloth coated in this manner exhibited blow-through pressures of20 psi whilst having an evaporation loss of 1500 grms/M² /24 hours.

EXAMPLE 4

To 100 grms of the fluoropolymer "Lumiflon" solution supplied by ICIMond Division, 8.5 grms of mono ethylene glycol was added again withvigorous stirring.

The resulting solution after de-aeration was applied to the cloth as forexample in FIG. 1 and heat treated for between 5 minutes at an airtemperature of 150° C. The coated sample exhibited a blow throughpressure of 35 psi with a water evaporation rate of 800 gms/m² /24hours.

We claim:
 1. A method of providing a water-repellant butwater-vapour-permeable coating over a void-containing substrate,comprising the steps of taking a solution consisting essentially of afluoropolymer in a mixture comprising at least a first organic liquidwhich is solvent thereof and a second organic liquid non-solvent for thefluoropolymer having a volatility lower than the first organic solventliquid and present in a ratio to the solvent liquid such that nofluoropolymer is precipitated; applying the solution to at least onesurface of the said substrate; and heating the substrate to drive offthe mixture of liquids as vapour, whereby initial loss of solvent liquidas vapour causes fluoropolymer to deposit within the voids of thesubstrate and over the surface thereof and subsequent loss ofnon-solvent liquid as vapour causes at least in part formation of poresthrough the deposited surface layer.
 2. A method as claimed in claim 1in which the fluoropolymer is chosen from the group consisting oftetrafluoroethylene/hexafluoropropylene/vinylidenefluoride copolymer, afluoropolymer solution including vinyl ethers produced by solutionpolymerization, and polyvinylidenefluoride polymer.
 3. A method asclaimed in claim 1 in which the first organic liquid is chosen from thegroup consisting of dialiphatic ketones, diaromatic ketones,alkylaromatic ketones, formamides and pyrrolidones.
 4. A method asclaimed in claim 3 in which the amount of polymer present in relation tothe first liquid is from 5 to 50 percent by weight.
 5. A method asclaimed in claim 1 in which the second organic liquid is chosen from thegroup consisting of aliphatic and aromatic hydrocarbons having 8 to 12carbon atoms or more, hydroxy-substituted aliphatic hydrocarbons such asdecanols or higher, and glycols and polyethylene glycols.
 6. A method asclaimed in claim 1 in which the second organic liquid is present in aratio of from 1 to 50 percent by weight of the solution of polymer inthe first organic liquid.
 7. A method as claimed in claim 1 furthercomprising the step of making up the liquid to be applied to thesubstrate by (a) dissolving the polymer in the first organic liquid andthereafter (b) adding the second organic liquid.
 8. A method as claimedin claim 1 in which the substrate is heated to a temperature between 60°and 150° C. for a period between 2 and 15 minutes.
 9. A method asclaimed in claim 1 applied to a solid structural or cladding material.10. A method as claimed in claim 1 applied to a protective leather orrubbery material.
 11. A method as claimed in claim 1 applied to afabric.
 12. A method as claimed in claim 1 applied to a made-up garment.13. A method as claimed in claim 4 applied to a solid structural orcladding material.
 14. A method as claimed in claim 4 applied to aprotective leather or rubbery material.
 15. A method as claimed in claim4 applied to a fabric.
 16. A method as claimed in claim 4 applied to amadeup garment.
 17. A method of providing a water-repellant butwater-vapour-permeable coating over a void-containing substrate,comprising the steps of taking a solution consisting essentially of afluoropolymer in a mixture comprising at least a first organic liquidwhich is solvent thereof and a second organic liquid non-solvent for thefluoropolymer having a volatility lower than the first organic solventliquid and present in a ratio to the solvent liquid such that nofluoropolymer is precipitated; applying the solution to at least onesurface of the said substrate; and heating the substrate to atemperature between 60° and 150° C. for a period between 2 and 15minutes to drive off the mixture of liquids as vapour, whereby initialloss of solvent liquid as vapour causes fluoropolymer to deposit withinthe voids of the substrate and over the surface thereof and subsequentloss of non-solvent liquid as vapour causes at least in part formationof pores through the deposited surface layer.